During the process of manufacturing memory circuits, as well as other semiconductor circuits, numerous copies of the circuits are manufactured simultaneously on a wafer. Each copy of the circuit is on a separate rectangular "die" on the wafer. The die on the wafer are each tested prior to packaging to determine which of the die should be packaged and which should be rejected. The die that pass all relevant tests are said to qualify for packaging.
When a memory array circuit, such as flash memory circuit, is being tested prior to packaging, the circuit is subjected to numerous tests before qualifying the circuit for packaging. One of the tests to which the memory array circuit can be subjected is a memory cell "IV" characteristics determination test, where "IV" stands for "current-voltage". The current-voltage characteristics of Individual cells are measured, for instance to determine the leakage current of "high threshold voltage" cells, which should ideally leak almost no current, to determine the current drawn by "low threshold voltage" cells, and to determine the threshold voltage of various memory cells in the circuit.
The standard way of measuring a memory cell's current is to connect a selected memory cell to a I/O pad, using circuits enabled only during a special testing mode of operation, and then to use an external current measurement circuit to measure the current drawn by the selected memory cell. The primary problem with this technique is that it is slow. More specifically, due to very small currents being measured (e.g., ranging from 5 to 200 microamps for a typical flash memory cell) and the nature of the external current measurement circuits used, measuring the memory cell current typically takes at least 100 microseconds, and sometimes as long as a few milliseconds. Since numerous current measurements may need to be taken on memory cells in each memory array, these types of measurements can significantly slow down die testing of memory array circuits.
It is therefore a goal of the present invention to provide a current measurement circuit and method for measuring memory cell currents much more rapidly than the external circuit measurement circuits described above, preferably in about 100 nanoseconds.
Another goal of the present invention is to provide a current measurement circuit and method for measuring memory cell currents that is inexpensive to implement.
Yet another goal of the present invention is to provide a current measurement circuit and method for rapidly measuring any specific device current in any semiconductor circuit.